Integrated thermal cracking and dehydrogenation process for olefin production

ABSTRACT

Embodiments disclosed herein relate to systems and processes for producing olefins and/or dienes. The systems and processes may include thermally cracking a C1-C4 hydrocarbon containing feed to produce a cracked hydrocarbon effluent containing a mixture of olefins and paraffins. The systems and processes may also include dehydrogenating the cracked hydrocarbon effluent to produce a dehydrogenated hydrocarbon effluent containing additional olefins and/or dienes.

BACKGROUND

To date, production of olefins from light paraffins (ethane, propane andbutane) have followed one of two major routes: non-catalytic thermalcracking in the presence of steam or catalytic dehydrogenation.

Ethylene plants are designed to maximize either ethylene or propylene orsum of ethylene and propylene. The thermal route produces a mixture oftargeted products (ethylene, propylene, butadiene/butenes). The mixturewill vary with the selected feed and the reaction conditions. Thequantity of butadiene produced in the olefins plant is generally low.Thermal crackers operate at relatively high temperatures and hence themajor product is ethylene.

On the other hand, dehydrogenation units, such as CATADIENE and CATOFINunits available from Lummus Technology LLC, dehydrogenate paraffins toolefins and olefins to dienes, and these units operate at relatively lowtemperatures. Hence, primary olefins (propylene from propane feed,normal butene from n-butane feed, and isobutene from isobutane feed) arethe main products. When normal butenes are present in the feed,butadiene is also produced. Since the equilibrium ethane conversion isvery low, the conversion of ethane to ethylene does not occur to asignificant level. In addition, dehydrogenation reactions are carriedout at low partial pressures either by operating the unit at lowabsolute pressure or in the presence of inert compounds. The feed todehydrogenation has to be heated to 500 to 650° C. to carry out thereaction to achieve economical conversions.

Historically, the route for a particular plant has been chosen based onthe available feed and the desired major product due to both energy andcapital cost considerations. For example, with butanes as the feed, thethermal cracking route will produce about 42% ethylene as the majorityproduct, but the catalytic dehydrogenation route will produce close to60% butadiene/butene product. Although reaction conditions can be variedto change these results, they do not change the target product nor dothey substantially change the yields.

SUMMARY OF THE DISCLOSURE

In one aspect, embodiments disclosed herein relate to a process forproducing olefins and/or dienes. The process may include thermallycracking a C1-C4 hydrocarbon containing feed to produce a crackedhydrocarbon effluent containing a mixture of olefins and paraffins. Theprocess may also include dehydrogenating the cracked hydrocarboneffluent to produce a dehydrogenated hydrocarbon effluent containingadditional olefins and/or dienes.

In another aspect, embodiments disclosed herein relate to a system forproducing olefins and/or dienes. The system may include a reaction zonefor thermally cracking a C1-C4 hydrocarbon containing feed to produce acracked hydrocarbon effluent containing a mixture of olefins andparaffins. The system may also include a dehydrogenation reaction zonefor dehydrogenating the cracked hydrocarbon effluent to produce adehydrogenated hydrocarbon effluent containing additional olefins and/ordienes.

In another aspect, embodiments disclosed herein relate to a process forproducing olefins and/or dienes. The process may include heating ahydrocarbon feedstock, comprising one or more C1-C4 hydrocarbons, in aconvection zone of a pyrolysis reactor to form a heated hydrocarbonmixture. The heated hydrocarbon mixture may then be mixed with steam, oranother inert, to form a mixed feedstock having a steam (inert) tohydrocarbon ratio in the range from 0.04 to 0.2. The mixed feedstock maythen be further heated in the convection zone of the pyrolysis reactorprior to reacting the mixed feedstock in a radiant zone of the pyrolysisreactor to convert a portion of the C1-C4 hydrocarbons to produce acracked hydrocarbon effluent containing a mixture of olefins andparaffins. The entirety of the cracked hydrocarbon effluent may then befed to a dehydrogenation reaction zone for dehydrogenating the crackedhydrocarbon effluent to produce a dehydrogenated hydrocarbon effluentcontaining additional olefins and/or dienes. The dehydrogenatedhydrocarbon effluent may then be separated to recover one or morefractions selected from a hydrogen fraction, a methane fraction, a C2fraction, an ethylene fraction, an ethane fraction, a C3 fraction, apropylene fraction, a propane fraction, a C4 fraction, a butadienefraction, a butene fraction, a butane fraction, and a C5+ containingfraction.

Other aspects and advantages will be apparent from the followingdescription and the appended claims.

BRIEF DESCRIPTION OF DRAWINGS

The FIGURE is a simplified process flow diagram illustrating anintegrated pyrolysis-dehydrogenation system for producing olefins fromhydrocarbon mixtures according to embodiments herein.

DETAILED DESCRIPTION

Embodiments disclosed herein relate generally to integrated processesfor the pyrolysis and dehydrogenation of hydrocarbon mixtures to produceolefins.

It has been found that an efficient integration of these two systems canprovide flexibility in both target products and total production ofvaluable products. Carrying out the pyrolysis reaction at first withsmall amount of steam or without steam, to low conversions, an optimizedfeed to the dehydrogenation unit may be obtained. This stream maycontain inerts (methane and other components which do not dehydrogenateeasily) and at the same time the feed is heated sufficiently for thedehydrogenation reaction. When this stream passes through thedehydrogenation catalyst, heavier molecular weight paraffins aredehydrogenated first. Since the activity of ethane dehydrogenation islow, ethylene to ethane (reverse reaction) is also low at thesetemperatures. As a result, the combined pyrolysis and catalyticdehydrogenation process produces ethylene, propylene, butenes andbutadiene all in higher concentration compared to individual pyrolysisor individual dehydrogenation alone.

The cracking reactor may be designed to operate at extremely lowconversion and in the presence of very low steam rates and/or theabsence of steam. This changes the thermal reactor design to a lowercost design and lower energy consumption design.

The products from the thermal reactor can be directly fed into thecatalytic dehydrogenation reactor system. The direct feed eliminates thepre-heating step from the conventional catalytic dehydrogenation reactorsystem. The system benefits from a mixture of reactive and non-reactive(or low reactivity) hydrocarbons in the feed resulting in improvedconversion, selectivity and total yield of valuable products. This alsorequires less catalyst in the catalytic reactor section.

For example, with butane feed, the total valuable products(ethylene+propylene+butadiene/butenes) will improve from a total ofabout 70% via conventional thermal cracker or about 65% via conventionalcatalytic dehydrogenation to over 80% for the integrated system.

The systems disclosed herein can also be designed to vary the splitacross the individual valuable products. The ability to vary feedcomposition, pyrolysis temperature, and dehydrogenation conditions mayallow great flexibility in the resulting product mixture.

The FIGURE illustrates a simplified process flow diagram of anintegrated pyrolysis and dehydrogenation system according to embodimentsherein. A pyrolysis heater 1, such as a fired tubular furnace, may beused for the thermal cracking of hydrocarbons to ethylene, propylene,butenes, butadiene and other olefinic or diene compounds. The firedtubular furnace 1 has a convection section or zone 2 and a crackingsection or radiant heating zone 3. The furnace 1 contains one or moreprocess tubes 4 (radiant coils) through which a portion of thehydrocarbons fed through hydrocarbon feed line 20 are thermally crackedto produce product gases upon the application of heat. Radiant andconvective heat is supplied by combustion of a heating medium introducedto the cracking section 3 of the furnace 1 through heating medium inlets8, such as hearth burners, floor burners, or wall burners, and exitingthrough an exhaust 10.

The hydrocarbon feedstock 20, which may be a single hydrocarbon or amixture of hydrocarbons, such as C1-C4 or C2-C6 hydrocarbons, may beintroduced to a heating coil 24, disposed in the convection section 2 ofpyrolysis heater 1 in the heating coil 24, the hydrocarbon feedstock maybe heated and/or vaporized via convective heat exchange with theexhaust.

If desired, the heated hydrocarbon feedstock 26 may then be mixed withsteam or an inert compound, such as nitrogen, carbon dioxide, or anyother inorganic gases. The dilution steam or inert may be supplied tothe process via flow line 28. Various portions of the process oradditional processes in the plant may use low temperature or saturatedsteam, while others may use high temperature superheated steam. Steam tobe used within the process or elsewhere in the plant may be heated orsuperheated via a heating coil (not shown) disposed in the convectionzone 2 of pyrolysis heater 1.

The heated hydrocarbon mixture in stream 29 may then be fed to a heatingcoil 30, which may be disposed at a lower elevation in the pyrolysisheater 1, and therefore at a higher temperature, than heating coil 24.The resulting superheated mixture may then be fed via flow line 32 toone or more coils (not labeled) in the one or more process tubes 4disposed in radiant zone 3 of pyrolysis heater 1, operated at atemperature for partial conversion, via thermal cracking, of thehydrocarbon mixture. The cracked hydrocarbon product may then berecovered via flow line 34.

The cracked hydrocarbon product may then be fed via flow lines 34, 36 todehydrogenation reaction zone 40. Optionally, additional hydrocarbons 38may be combined with cracked hydrocarbon product 34 for conversion inthe dehydrogenation reaction zone 40. The additional hydrocarbons mayinclude, for example, additional C2-C4 or C5 hydrocarbons, including aportion of hydrocarbon feedstock 20, for example. Depending upon theoutlet temperature of the one or more coils in the one or more processtubes 4, the additional hydrocarbons may be used to cool, but notquench, the cracked effluent to a desired dehydrogenation reaction zoneinlet temperature.

The hydrocarbons in flow line 36 may then be forwarded todehydrogenation reaction zone 40, which may include one or moredehydrogenation reactors 61 operating in series or in parallel, asillustrated. The dehydrogenation reactors may each contain one or morebeds 63 containing a dehydrogenation catalyst. A dehydrogenated effluentmay then be recovered from reactors 61 via flow lines 65 and forwardedvia flow line 67 to a product recovery and separation zone (notillustrated).

While illustrated and described above with respect to a pyrolysisheater, the thermal cracking of the feedstocks may also be performed inother types of heaters.

As described above, embodiments herein integrate pyrolysis withdehydrogenation. Carrying out the pyrolysis reaction first, with a smallamount of steam or without steam, to low conversions, an optimized feedto the dehydrogenation unit is obtained. This stream may contain inertcompounds (such as methane and other components which do notdehydrogenate easily) and at the same time the feed is heatedsufficiently for the dehydrogenation reaction.

When this stream passes through the dehydrogenation catalyst, heaviermolecular weight paraffins are dehydrogenated first. Since the activityof ethane dehydrogenation is low, ethylene to ethane (reverse reaction)is also low at these temperatures. As a result, the combined pyrolysisand catalytic dehydrogenation process produces ethylene, propylene,butenes, and butadiene, all in higher concentration compared toindividual pyrolysis or individual dehydrogenation alone.

For example, when 100% normal butane is thermally cracked, approximately20 wt % CH4, 42% C2H4, 18% C3H6, 5% C4H6, 4% C4H8 and 1.2% H2 (thebalance other products) will be obtained at high severity. When 100%n-butane is catalytically dehydrogenated 1.3% C2H4, 4.3% C3H6, 46%normal butenes, 12.5% C4H6 and 0.6% H2 (balance other products) areobtained. These are illustrative values only.

In contrast, embodiments herein may first thermally crack n-butane atlow conversions (about 50%) followed by catalytic dehydrogenation, whereboth ethylene and propylene yields are increased. Unconverted butane inpresence of these inerts reduces the effective partial pressure, whichfavors the dehydrogenation equilibrium to butenes and hencedehydrogenates to butenes and butadienes favorably. The pyrolysiseffluents act as diluents for this reaction, lowering the effectivehydrocarbon partial pressure of the main reactants and also suppressingcoke formation. As a result, the calculation shows about 16% ethylene,22% propylene and 43% butene+butadiene can be expected. Compared tocatalytic dehydrogenation alone, butene+butadiene yields will be low.However, the sum of ethylene+propylene+butenes+butadiene (so called highvalue products) will be high compared to either pyrolysis or catalyticdehydrogenation alone. Energy consumption of embodiments herein willalso be low. Since the energy used to preheat the feed for catalyticdehydrogenation is used with a small additional energy in the sameheater to carry out the pyrolysis reaction, energy consumption is low. Asingle separation train may be used for product recovery. Therefore, theintegrated scheme reduces the capital cost and reduces the energyconsumption. Any light hydrocarbon feed up to C4 can be used in thisscheme. This also reduces the feed preparation cost since pure butane orpropane need not be produced for feed to the catalytic dehydrogenationreactor.

Judicial combination of these processes and operating at optimumconditions may thus produce a maximum amount of required olefins anddiolefins with minimum amount of less valuable byproducts. Theindividual processes produce less quantity of olefins than theintegrated process for a given quantity of C3 or C4 feed. Instead ofn-butane as given in the example, one can use propane and/or propaneplus butane mixtures. If isobutene is not required, isobutane can bepositionally isomerized to form n-butanes and used as feed.Alternatively, isobutene product can be positionally isomerized tonormal butenes, such as by using CDISIS technology, available fromLummus Technology LLC.

The above example was illustrated with 100% n-butane used as feed.Embodiments herein may use any feed, including those containing olefins,where the feed may include C1 to C4 hydrocarbons. In other words,feedstocks herein may include methane, ethane, propane, and/or butane(normal and/or iso), alone or in combinations of two or more of these.Feedstocks herein may also contain olefins, such as ethylene, propylene,and butenes. Feeds having C5+ impurities can also be used. CO2, N2 andsteam, or any other inert gases, may be used as a diluent to reduce thepartial pressure and increase olefin selectivity in the pyrolysis andcatalytic dehydrogenation reactors.

In some embodiments the feed is first preheated, such as in a preheaterused for heating feed to a catalytic dehydrogenation reactor. Thisheater may act as a low severity pyrolysis reactor. Alternatively, aconventional pyrolysis heater can be used for this step (as shown anddescribed for FIG. 1). The thermal cracking reaction may be carried out,for example, at a temperature greater than is required for typical feedpre-heat to a catalytic, dehydrogenation reactor, so as to achieve thedesired thermal cracking conversion. Alternatively, the thermal crackingreaction may occur over a longer residence time than is typical forpre-heat to a catalytic dehydrogenation reactor, so as to achieve thedesired conversion. In other words, the integrated processes andbenefits herein are not accomplished by prior art catalyticdehydrogenation processes simply by including a preheater to warm up thefeed to catalytic dehydrogenation temperatures, where the typicalpreheat results in essentially no or minimal (<1%) conversion.

Typically, the pyrolysis reaction is carried out in the presence ofsteam. Since the conversion targeted in embodiments herein is relativelylow, a small amount of steam (0.04 to 0.2 wt/wt, such as from 0.05 to0.15 wt/wt) may be used. In some cases, the pyrolysis reaction may becarried out in the absence of steam. When inert compounds like methane(CH4) are present in sufficient quantities in the feed, a lower steam tohydrocarbon (also referred to as steam to oil, S/O) ratio can be used,and as noted earlier, may be as low as 0.

Depending upon the feed composition, a suitable low conversion may beused for the thermal cracking step. In some embodiments, a suitableconversion may be less than 10%, 10-20%, 20-30%, 30-40%, 40-50%, 50-60%,60-70% and combinations thereof inclusive of all points therebetween. Insome embodiments, for example, the conversion may be in the range fromabout 10% to about 70%. For para n-butane feed, for example, the targetconversion will be less than 70%, such as less than 60%, or less than55%, or less than 50%, and in some embodiments may be around 50% orbetween 40% and 60%. For propane rich feeds, as another example, thetarget conversion in the thermal cracking step may be less than 60%,such as less than 50%, and in some embodiments may be less than 40%,such as between a lower limit of 20% or 25% or 30% to an upper limit of35% or 40% or 50%. Conversions noted herein are on a molar basis unlessotherwise noted.

Depending upon the feedstock used, the effluent from the pyrolysisreactor may contain H2, CH4, C2H4, C2H6, C3H6, C3H8, C4H6, C4H8, C4H1Oand C5 plus. The conversion in the pyrolysis reactor may be controlledin some embodiments such that the concentration of C5+ material in thedehydrogenation reactor feed is low, such as less than 0.5 wt %, lessthan 1 wt %, less than 2 wt %, or less than 5 wt %, for example.

The maximum coil outlet temperature for the pyrolysis step should bekept relatively low. Radiant coil outlet temperatures (COT) may be lessthan 725° C., for example, and less than 675° C. is preferred. In someembodiments, the COT may be in the range from about 550° C. to about725° C., such as from about 600° C. to about 700° C.

When higher coil outlet temperatures are used, a cold feed can beinjected to cool the pyrolysis effluent to a desired temperature priorto catalytic dehydrogenation. Very high temperatures may affect theperformance of the catalytic dehydrogenation catalyst. Based on thecatalyst being used, the dehydrogenation feed temperature should beappropriately selected. This temperature has to be sufficient enough forcarrying out the dehydrogenation reaction. Typically, this temperature(inlet to catalytic dehydrogenation reactor) is in the range from about500° C. to about 650° C., such as in the range from about 550° C. toabout 650° C. or in the range from about 500° C. to about 600°. Theproducts coming out of the pyrolysis reactor are not componentiallyseparated prior to catalytic dehydrogenation (as compared to separationinto two or more parts of equal composition, such as via a piping teefor feeding parallel reactors).

Pyrolysis of propane and butane produce methane as a byproduct.Therefore, methane and other non-reactive species may act as a diluentfor the catalytic dehydrogenation reaction. Depending upon theconversion level targeted in the dehydrogenation reactor(s), thedehydrogenation reaction can be carried out at vacuum or slightly aboveatmospheric pressure. The corresponding pyrolysis operating pressure maybe chosen accordingly to facilitate flow from the pyrolysis reactor(s)to the dehydrogenation reactor(s).

The combined pyrolysis pre-reactor and catalytic dehydrogenation reactorcan be used in a variety of catalytic dehydrogenation reactorconfigurations, such as cyclic reactors, continuous fixed bed reactors,moving bed reactors or fluidized bed reactors. For example, in a cyclicreactor operation (such as one having a 7 to 15 minute cycle), enoughswitching valves from pyrolysis to the dehydrogenation section may beprovided to facilitate the transition of pyrolysis effluent flow betweenthe reactors, as well as to provide the purge and regeneration flows.The dehydrogenation reactor will operate by switching to differentreactors every 7 to 15 minutes in the cycle, followed by purge andregeneration steps prior to coming on line for dehydrogenation again.Instead of a cyclic reactor system, other fixed bed, ebullated, moving,or fluid bed dehydrogenation reactors may also be used. Embodimentsherein are not limited to a particular type of dehydrogenation reactorsor reactor scheme, and may include multiple types of reactors, which maybe in parallel and/or series configurations.

The effluent from the dehydrogenation reactors may be processed toappropriately separate and recover the olefins and dienes. For example,one or more distillation towers may be used to separate thedehydrogenation reactor effluent into two or more fractions, such as ahydrogen fraction, a methane fraction, a C2 fraction, an ethylenefraction, an ethane fraction, a C3 fraction, a propylene fraction, apropane fraction, a C4 fraction, a butadiene fraction, a butenefraction, a butane fraction, and/or a C5+ containing fraction. Ifdesired, a portion or all of the C5+ containing fraction(s) may berecycled for further cracking to produce additional C2-C4 olefins.

As described above, embodiments herein may produce olefins and dienesvia an integrated pyrolysis and catalytic dehydrogenation process.Embodiments herein may produce a higher sum of ethylene, propylene,butene, and butadiene products compared to either pyrolysis ordehydrogenation alone. Further, the energy consumption will berelatively low, as only a small additional energy is needed compared topreheating the feed for catalytic dehydrogenation. Therefore, theintegrated scheme reduces the capital costs and reduces energyconsumption for the production of valuable olefins and dienes.

While the disclosure includes a limited number of embodiments, thoseskilled in the art, having benefit of this disclosure, will appreciatethat other embodiments may be devised which do not depart from the scopeof the present disclosure. Accordingly, the scope should be limited onlyby the attached claims.

What is claimed:
 1. A process for producing olefins and/or dienes, theprocess comprising: thermally cracking a C1-C4 hydrocarbon containingfeed to produce a cracked hydrocarbon effluent containing methane and amixture of C2-C4 olefins and paraffins, wherein a conversion ofhydrocarbons in the hydrocarbon containing feed is in a range from about10 mol % and to 70 mol %; without componential separation of the crackedhydrocarbon effluent, dehydrogenating the cracked hydrocarbon effluentto produce a dehydrogenated hydrocarbon effluent containing additionalolefins and/or dienes.
 2. The process of claim 1, further comprisingcooling the cracked hydrocarbon effluent via direct heat exchange with ahydrocarbon feed containing one or more C1-C4 hydrocarbons.
 3. Theprocess of claim 1, wherein the thermally cracking is performed in oneof a pyrolysis reactor or a heat exchanger, wherein the crackedhydrocarbon effluent recovered from the pyrolysis reactor or heatexchanger is at a temperature in a range from about 550° C. to about725° C.
 4. The process of claim 3, further comprising cooling thecracked hydrocarbon effluent to a temperature in a range from about 500°C. to about 650° C. prior to dehydrogenating the cracked hydrocarboneffluent.
 5. The process of claim 1, further comprising separating thedehydrogenated effluent into one or more fractions selected from ahydrogen fraction, a methane fraction, a C2 fraction, an ethylenefraction, an ethane fraction, a C3 fraction, a propylene fraction, apropane fraction, a C4 fraction, a butadiene fraction, a butenefraction, a butane fraction, and a C5+ containing fraction.
 6. Theprocess of claim 1, wherein the C1-C4 hydrocarbon-containing feedcomprises n-butane, the process further comprising controlling thethermally cracking step to a conversion in the range from about 40 mol %to 60 mol %.
 7. The process of claim 1, wherein the feed C1-C4hydrocarbon-containing comprises propane, the process further comprisingcontrolling the thermally cracking step to a conversion in the rangefrom about 25 mol % to 40 mol %.
 8. The process of claim 1, furthercomprising mixing steam, carbon dioxide, and/or nitrogen with the C1-C4hydrocarbon-containing feed to form a diluted feed mixture.
 9. Theprocess of claim 8, wherein the diluted feed mixture has a diluent tohydrocarbon ratio in the range from about 0.04 to 0.2, by weight. 10.The process of claim 1, further comprising conducting the thermalcracking step at a pressure greater than the dehydrogenation.
 11. Theprocess of claim 1, further comprising conducting the thermal crackingstep at a coil outlet temperature greater than a feed inlet temperatureof the dehydrogenation.
 12. The process of claim 1, wherein the C1-C4hydrocarbon-containing feed comprises isobutane, the process furthercomprising positionally isomerizing the isobutane to form n-butanes. 13.The process of claim 1, wherein the C1-C4 hydrocarbon-containing feedcomprises isobutane, the process further comprising positionallyisomerizing isobutene in the dehydrogenated hydrocarbon effluent to formn-butenes.
 14. A process for producing olefins and/or dienes, theprocess comprising: heating a hydrocarbon feedstock, comprising one ormore C1-C4 hydrocarbons, in a convection zone of a pyrolysis reactor toform a heated hydrocarbon mixture; mixing the heated hydrocarbon mixturewith steam to form a mixed feedstock having a steam to hydrocarbon ratioin a range from 0.04 to 0.2; heating the mixed feedstock in theconvection zone of the pyrolysis reactor; reacting the mixed feedstockin a radiant zone of the pyrolysis reactor to produce a crackedhydrocarbon effluent containing methane and a mixture of olefins andparaffins; feeding the entirety of the cracked hydrocarbon effluent,without componential separation, to a dehydrogenation reaction zone fordehydrogenating the cracked hydrocarbon effluent to produce adehydrogenated hydrocarbon effluent containing additional olefins and/ordienes and; separating the dehydrogenated hydrocarbon effluent torecover one or more fractions selected from a hydrogen fraction, amethane fraction, a C2 fraction, an ethylene fraction, an ethanefraction, a C3 fraction, a propylene fraction, a propane fraction, a C4fraction, a butadiene fraction, a butene fraction, a butane fraction,and a C5+ containing fraction.
 15. The process of claim 14, wherein thefeedstock comprises n-butane, the process further comprising controllingthe thermal cracking step to a conversion in a range from about 40 mol %to 60 mol %.
 16. The process of claim 14, wherein the feedstockcomprises propane, the process further comprising controlling thethermal cracking step to a conversion in a range from about 25 mol % to40 mol %.
 17. The process of claim 14, further comprising controlling acoil outlet temperature of the cracked hydrocarbon effluent from thepyrolysis reactor to a temperature in a range from about 660° C. toabout 725° C.
 18. The process of claim 17, further comprising coolingthe cracked hydrocarbon effluent, via direct heat exchange with ahydrocarbon feed comprising one or more C1-C4 hydrocarbons, to atemperature in a range from about 550° C. to about 650° C., prior todehydrogenating the cracked hydrocarbon effluent.